WH are tested for pressure. While they may test them for vacuum, I've never seen it listed. Get the right conditions, and you can get a vacuum in a WH. It doesn't take much to collapse or distort a vessel designed for pressure when you substitute vacuum - the tensile strength is generally a lot stronger than the crushing into a void.

thought I would let everyone know that no one over there at PZ has ever actually seen an imploded water heater...

but some of them claim that they have seen big foot, and a couple of them claim that they have been abducted by aliens too...

but no imploded heaters

lots of them claim that they have to install the vrv valves on heaters in their area where the heater is above the fixtures on the first floor.....which was a suprise to me.....

In this city and surrounding counties their are at least 150,000++ homes built since the mid 90s with heaters on the second floor and no vrv valves on anything.....but nothing has ever gone wrong that I have heard about.... basically no imploded heaters

I suppose the inspectors in this State must think they are not necessary because of the hole in the dip tube and feel that its a complete waste of time and money.-

or we are just a bunch of ass-backwards hicks living out in the sticks

I think that you could find that hj and nhmaster3015 (he hasn't been on this forum for a couple of years) have already expressed that same concerns and situations for vacuums in hot water tanks on other plumbing forums and they also meet resistance from other plumbers.
A common situation that simulates what can happen to a tank is this; A customer has a leaking hot water tank. They shut off the gas and the cold water supply and the tank is leaking onto the floor while they wait for a plumber. When the plumber arrives hours later or the next day, the homeowner explains that it wasn't leaking badly but then after several hours they heard a WHOOSH!! and the tank suddenly blew loose at the bottom and all of the water came dumping out. What happened was as the tank emptied there was a void created at the top and the water was cooling. Once the water got low enough and the air in the void cooled enough a vacuum occurred and compressed the remaining water finding the weakest spot on the tank (the leak) and blew the hole open wider forcing the water out. The nipples and sometimes even the pipes connecting are all bent up from the action. This same situation is created with homeowners who flush their tanks or plumber who do repairs on water piping. It's commonly mistaken as an exploded tank and some plumbers on other forums even blame it on thermal expansion. Tanks can over heat and blow the jacket of the tank wide open, but on imploded tanks there is usually little external evidence (the jacket appears to be normal) only showing the distorted nipples and pipes mentioned above.

This same situation is created with homeowners who flush their tanks or plumber who do repairs on water piping. It's commonly mistaken as an exploded tank and some plumbers on other forums even blame it on thermal expansion. Tanks can over heat and blow the jacket of the tank wide open, but on imploded tanks there is usually little external evidence (the jacket appears to be normal) only showing the distorted nipples and pipes mentioned above.

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that sounds like a great theory, but I do not believe in big foot either...

The picture of the unit I posted earlier was a tank about to explode....
the nipples on top were crushed inwards because the tank itself had expanded
outwards dramatically like a baloon....

If a water heater was about to crush in on itself I would think
that the nipples would be pointing outwards, of course we is getting
into some grey area of thermo-dynamics here ....

So The next time I see a heater with the nipples pointing outwards,
I will cut the jacket off of it and see for myself....and I will post it here.

this will probably happen some time after I post the picture of bigfoot
walking across my back yard.....

the pz site someone claims that a marathon plastic heater imploded
and cracked the liner due to someone draining it and voided the warranty
but no one has actually seen an imploded heater....

I will see my Rheem salesman this morning and ask him about this whole thing
I wonder if he is totally in the dark about this also...

By what you claim, I actually ought to be at ground zero for imploded water heaters in INDY cause not one plumber in town has ever installed a vrv valve on heaters in tract homes on the second floor since at least 1990. I know that there are well over 150+++ heaters out there on second floors in this region....

so BIG-Foot has to be out there just waiting for me to find him......and I am gonna get me a picture of one of these critters some day

This is what an imploded hot water heater looks like. Nipples and T&P valve bent inward. If you ever come across this, remove the jacket. It will appear to still be intact from the outside. Once removed you will easily see that it is not outward force, but inward. This happened suddenly to a member of another forum and this is a mild case of imploding tanks.
If I find some with more detail or a jacket removed I will post them.
This is the post from the thread describing the situation:

"I seen a few things in my day but this one is a first for me. My daughter called to say she was worried about her hot water heater. I thought she was over reacting a bit and had just seen too many insurance commercials! You know, the hot water heater missile. She tried to convince me that her hot water heater's (that been known to drip a bit) down pipe had moved and was sticking out at an angle. I told her not to worry and I'd be over to look at it the next day. She did say as a bonus that the dripping had stopped though.

Well, it appears that the hot water heater is quite literally collapsing from top. The metal of the hot water heater itself seems to sunken in. And when the metal top of the hot water heater sunk in it caused the pipes to move as well and they're now at strange angles.

And, yes she right, because it's sunk, the down pipe is sticking out at quite an angle. See picture. This has happened at some point in the last week or so.

She bought this small 1950's bungalow about 1.5 yrs. ago and it is a manufactured pre-fab home (National Homes) on a traditional block foundation.

Has anyone ever encountered this before or have any idea why it would have done this. Suffice it to say we'll be replacing it very soon. "

It's been almost 24 hours since I posted the pictures. Isn't somebody going to jump on this thread, breathe some life into it and tell me that the pictures that I posted are something other than what I stated? C'mon...it's not THAT challenging.

Okay. Let's assume it has imploded. If so, how could it be? It appears that the WH is in the basement. You won't siphon the water out of it (unless you turned off gravity and flipped it around). If everything was closed off and the WH was drained from the bottom, the most vacuum that could ever be generated would be something like the elevation head of the WH itself (maybe 1-2psi at best) without some active means. It also sounds like nothing out of the ordinary was taking place when this happened.

I have not heard that one in a long time....
Inside most heaters today there is a port hole on the
dip tube inlet at the top of the tank that prevents the
heater from siphoning....... a heater cannot siphon down

the valve they want you to install is not necessary

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I am new here just a lady who had an Hvac guy install a Rheem waterheater I realize this is an older thread but I learned so much valuable info from it. I hope you pros can help me but the installer said he could give me a Watts vrv as a little extra on my hw tank for 25$ & I said yes OK. I Googled it afterward & since this Rheem Classic 40 gal gas tank is installed in my basement & water connections are top feed I don't understand why I need this valve. What can I do? Is there anything I should know about turning water off if I go away or risk of it failing? Frankly it seems weird he put it on there. Not reqired here. Sorry to bother you but can anyone advise? Could it be taken off? It seems like more leak or fail risk & maybe harder to repair tank in future from reading this thread?

I would leave it on. It prevents damage from water being turned off and the tank draining without an open spigot.

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OK thank you very much I was a bit worried after all the posts about them. Sounds like fine to turn off the water for awhile while I am away or get a repair when necessary then. I guess I feel like just one more unnecessary part to fail. Also tiny bit of water on joint of water pipe it's on just a tiny drop every once in a while so if need that fixed would it be a problem with the vacuum? Would I know if it was starting to fail before any mass spewing of water? If I did want to replace it is it a problem to install a plug instead?

For what it's worth, it is industry standard and best practices here to install a vacuum relief valve(like a watts N36) on all hot water tank replacements. The city requires them(along with thermal expansion protection) on new builds and permitted renovations. The code simply says they are required anywhere a tank may be subject to back-siphonage; the inspectors often interpret this to be anywhere -any- part of the city water system is below the level of the hot water tank.

Vacuum relief valve generally do not fail catastrophically(spewing). There is no issue shutting the system down or repressurizing, though they should be inspected after turning the system back on. An older valve may weep a drop or two during system start-up. The way they usually fail is keep weeping after the system has reached normal pressure. I've heard product reps suggest a 15 year life span, and though I've seen many much older, this means most plumbers will recommend replacing the vacuum relief valve when the hot water tank is replaced. Installing a plug is not recommended.

you dont need one unless your heater is fed from the bottom....like it says
in the section 504.... They DONT MAKE a heater that feeds from the bottom
unless you are talking about 6 gallon electric heaters .....

You will be ok... just make sure they install the heater in a catch pan going to
the nearest drain...that is much more important than a silly vaccuum breaker